1. Technical Field
The present invention relates to an ink composition preferably used in ink jet recording, an ink jet recording method, a planographic printing plate obtained by using the ink composition, and a method of producing a planographic printing plate. In particular, the invention relates to an ink composition that is preferably used in ink jet recording and that cures by irradiation of active radiation rays with high sensitivity to form a cured matter having sufficient flexibility, an ink jet recording method, a planographic printing plate obtained by using the ink composition, and a method of producing a planographic printing plate.
2. Related Art
Examples of methods for forming an image on a recording medium such as paper based on an image data signal include an electrophotographic method, a sublimation-type thermal transfer method, a melt-type thermal transfer method, and an ink jet recording method. The electrophotographic method requires a process of forming an electrostatic latent image on a photoreceptor drum by charging and light exposure, and a system therefore becomes complicated, resulting in increased production cost. The thermal transfer method can be conducted by an inexpensive apparatus, but requires use of ink ribbons, leading to increased running cost and generation of waste.
The ink jet recording method can also be conducted by an inexpensive apparatus. Moreover, an image is directly formed by ejecting ink only to regions of a support which are to be image regions, and the ink can therefore be efficiently used, resulting in reduced running cost. Further, ink jet recording apparatuses are not noisy. Accordingly, the ink jet recording method is an excellent image forming method.
There is a demand for an ink composition, such as an ink composition for ink jet recording, which cures by irradiation of active radiation rays such as ultraviolet rays with high sensitivity to form high-quality images (radiation-curable ink composition). Improvement in sensitivity of ink compositions with respect to irradiation of active radiation rays increases the curing property of the ink compositions, leading to reduced power consumption and reduced load applied to an active radiation ray generator, which lengthen the lifetime of the generator. Moreover, sufficient curing suppresses vaporization of uncured low-molecular weight substances and deterioration in strength of images formed. In addition, when the ink composition is used to form image regions of a planographic printing plate, improved strength of images due to improved sensitivity brings increased printing durability to the image regions.
An ink jet method involving UV curing has been attracting attention recently, since the method generates relatively weak odor and enables images to be recorded on recording media that do not have a quick drying property or an ink absorbing property. In such a method, benzil, benzoin, benzoin ethyl ether, Michler's ketone, anthraquinone, acridine, phenazine, benzophenone, or 2-ethylanthraquinone is generally used as a photopolymerization initiator (see Chemical Review, Vol. 93, pp. 435-448, written by Bruce M. Monroe et al., and published in 1993). However, a photopolymerizable composition including such a photopolymerization initiator, which has low sensitivity, requires a long time for image-wise exposure in forming images. Therefore, in a case where the image to be formed is minute, if there is slight vibration during the operation, images with good image quality cannot be obtained. Further, since a long exposure time is accompanied by an increase in energy radiation from the exposure light source, a measure against radiation of the great heat resulting therefrom is needed.
For the purpose of providing an ink composition that is highly safe, causes little skin irritation, has low sensitizing property, and allows printing of an image having high sensitivity and strong adhesiveness even on a support (recording medium) on which it is usually difficult to form an image with no ink bleeding by an ink jet recording method, compositions containing polymerizable compounds that are particular acrylate compounds have been proposed, (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2003-192943). However, since a polymer obtained by curing a multifunctional acrylate used to improve sensitivity has a highly cross-linked three-dimensional structure, an image obtained by curing an ink including such a multifunctional acrylate has deteriorated flexibility and impact resistance, and therefore easily cracks on the recording medium or peels off, and has deteriorated durability.
Conventionally, so-called PS plates each having an oleophilic photosensitive resin layer on a hydrophilic support are used, and the photosensitive resin layer is image-wise exposed to light to form exposed regions having increased or decreased solubility in an alkaline developing solution, and non-image regions are dissolved in the alkaline developing solution and removed in preparing planographic printing plates. However, along with recent broad dissemination of digitization technology in which image information is electronically processed, stored and output with a computer, there is a need for a new image output method that corresponds to the digitization technology. In particular, methods of producing a printing plate without use of a developing solution are under study, and a method in which a planographic printing plate is directly prepared by using an ink composition for ink jet recording has been proposed (see, for example, JP-A No. S54-117203). In this method, a desired image, which is preferably hydrophobic, is formed by image-wise ejecting an ink on the surface of a support, which is preferably hydrophilic, in an ink jet recording manner and irradiating the ink with active radiation rays to cure the ink. In order to form image regions of a planographic printing plate, it is preferred that ink droplets deposited on the support cure rapidly without bleeding, the cured image regions have excellent strength and strong adhesiveness to the support, and, when the planographic printing plate is being loaded in a printing machine, the image regions conform to the deformation of the support without generation of damage such as cracks. Accordingly, there is currently a need for an ink composition suitable for such application.
Thus, there is a need for an ink composition that is cured with high sensitivity by irradiation of active radiation rays and, when cured, provides a high-quality and sufficiently flexible image; and an ink jet recording method using the ink composition.
There is also a need for a planographic printing plate prepared by using an ink composition that can be cured with high sensitivity by irradiation of ultraviolet rays; and a method for producing such a planographic printing plate.